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We present tailoring of the performances of thin multiplication layer InAlAs/InGaAs avalanche photodetectors (APDs) with operating voltages lower than 20 V. Their operating voltages, gain-voltage slopes and dark currents were successfully tailored by changing the electric field distributions in avalanche region. The thin multiplication layer APDs show small activation energies of the dark current ranging from 0.12 to 0.19 eV at temperatures above 220 K, suggesting a band-trap-band tunneling dominant dark current mechanism over this temperature range. The dark currents show very weak temperature dependences at temperatures lower than 175 K, which mainly originate from the band-to-band tunneling and the surface leakage currents. The spectral responsivity of those APDs show anomalous negative temperature coefficients at gain factors larger than 1, which is attributed to the enhanced phonon scattering effect of carriers in the avalanche region at higher temperatures. Good gain factor uniformity at a given bias is observed for those APDs, and the charge layer is found to help improve the gain uniformity.
© 2015 Optical Society of America
1. Introduction
Focal plane arrays (FPAs) based on In0.53Ga0.47As (referred as InGaAs hereafter) avalanche photodetector (APD) elements show an attractive application prospect in infrared imaging area which has been also regarded as one of the 4th generation infrared detection systems [1]. Benefiting from the internal gain of pixels, APD-FPAs provide a ultra-high imaging sensitivity which is particularly suitable for detection in photon-starving environments, e.g. space remote sensing and laser radar (LADAR) imaging [2, 3]. In those applications, low operating voltage, small gain-voltage slope and high APD pixel gain uniformity are highly appreciated to improve the voltage compatibility for readout integrated circuits [4, 5]. Besides, low dark currents (Id) are also crucial to maintain a high signal to noise ratio. Thin multiplication layer is generally exploited to lower the operating voltage of APDs taking advantage of the carrier’s ’dead-space effect’ [6–8 ]. The carrier transit time in the thin avalanche region is shortened which helps increase the APD’s bandwidth. On the other hand, as a result of the nonlocal nature of impact ionization, the excess avalanche noise of APDs decreases with decreasing thickness of multiplication region [9, 10]. However, as a trade-off, Id increases in thin multiplication layer APDs [8,11,12] since in order to achieve a same gain factor (M) as in thick multiplication layer APDs the electric (E) field in the thin avalanche region must be increased, which leads to a notable increase of band tunneling leakage in multiplication layer.
Essentially, the gain characteristics of APDs are predetermined by the local E field intensity, the E field distribution and the carrier impact ionization threshold energy. This allows control of the operating voltage, the M and the gain-voltage slope via tailoring the E field profiles in the avalanche region. The threshold energy is an intrinsic parameter which directly depends on the material itself in multiplication layers. In0.52Al0.48As (referred as InAlAs hereafter) has been demonstrated to be a good electron multiplication material for InGaAs separate absorption and multiplication (SAM) APDs owing to its modest electron impact ionization threshold energy of 1.9–2.2 eV, a higher ionization coefficient ratio of electron to hole than that of hole to electron in InP, and a small excess noise factor. [9, 13] By adjusting the doping, the layer thickness and the layer architectures of the separate absorption, grading, charge and multiplication (SAGCM) InAlAs/InGaAs APDs, both wedge-shaped [14] and uniformed [15, 16] E fields have been demonstrated, showing improved gain-voltage slope or gain-bandwidth product (GBP). Small gain-voltage slope of APDs allows maintaining a more stable gain over a voltage fluctuation range. To achieve APDs with low operating voltage but small gain-voltage slope, the multiplication layer needs to be thin whereas the operating voltage range must be as large as possible. Our previous work has demonstrated the possibility of lowering the gain-voltage slope of thin multiplication layer InAlAs/InGaAs APDs by introducing an E gradient in the avalanche region [14].
In this work, we demonstrated the tailoring of the performances of thin multiplication layer InAlAs/InGaAs SAGCM APDs. The operating voltages, Id as well as gain-voltage slopes were tailored by controlling the E field distributions in avalanche region. Small gain-voltage slopes and low operating voltages less than 20 V with relatively low Id are realized. The Id is extensively studied and the dominating mechanisms in different temperature ranges are identified. Moreover, the temperature dependent behaviors of the spectral responsivity of those APDs at M larger or less than 1 are found to show opposite characteristics, which are explained by the enhanced phonon scattering effect of carriers in the avalanche region at high temperatures. The M uniformity at a given bias as well as its possible origins is also investigated.
2. Experimental details
The InAlAs/InGaAs APDs are SAGCM-type devices and were grown on n+-type (sulfur-doped) InP(100) substrates in a VG Semicon-V80H gas-source molecular beam epitaxy (GSMBE) system. N- and p-type dopants are silicon and beryllium, respectively. The epitaxial layer structure is sketched in Fig. 1(a), which in turn consists of heavily n-doped 200-nm-thick InP (2×1018 cm−3) and 800-nm-thick InAlAs (4×1018 cm−3) buffer layers, a thin p-doped InAlAs multiplication layer, a 70-nm-thick heavily p-doped (6×1017 cm−3) InAlAs charge layer, a 72-nm-thick p-doped (6×1016 cm−3) InAlGaAs grading layer, a 1.5-μm-thick p-doped (6×1016 cm−3) InGaAs absorption layer, a 450-nm-thick heavily p-doped (3×1018 cm−3) InAlAs electron diffusion blocking layer and a 150-nm-thick heavily p-doped (3×1018 cm−3) InGaAs contact layer. To tailor the E field profiles in the avalanche region, three APD wafers denoted by APD-1, APD-2 and APD-3 were grown with multiplication layer thicknesses of 300, 150 and 150 nm, respectively. The doping densities of APDs 1, 2 and 3 are 3×1017, 1×1017 and 6×1016 cm−3, respectively. In order to realize a wedge-shaped E profile, the charge layer was removed for APD-1. The material parameters as well as the measured device performances of the three APDs are listed in Table 1. The wafers were processed into top-illuminated square mesa-type photodiodes without anti-reflection (AR) coating using standard photolithography, chemical wet-etching, Si3N4 passivation and lift-off metallization techniques. The top InGaAs contact layer in the photo-sensitive area was removed using chemical wet-etching in a mixed solution of citric acid and H2O2 after processing to eliminate photo absorption therein. A 1550 nm laser diode with an attenuated output power of 5 μW from a single-mode fiber was used as light source in the photocurrent measurements. The temperature dependent dark current-voltage (Id-V) and spectral responsivity characteristics were measured by bonding the device into a TO-56 package and then mounting in an opened cycle liquid nitrogen cryostat, which controls temperature in the range of 77–350 K.
Fig. 1 (a) Schematic epitaxial layer structure of APDs 1, 2 and 3. (b) Simulated E field distributions along the growth direction for the three APDs at a same M of 2.
Table 1. A list on the material parameters and the measured device performances of the three APDs. ’M-layer’ denotes ’multiplication layer’.
3. Results and discussions
The actual room temperature (RT) carrier concentrations in the p-InAlAs multiplication layer of the three APDs are measured via capacitance-voltage (C-V). Results show neglectable differences with the nominal doping densities. Figure 1(b) shows the simulated E field profiles along the growth direction at RT for the three APDs at a same M of 2. The voltages at M=2 for APD-1, APD-2 and APD-3 are determined to be −16.5, −14.2 and −16 V based on the photocurrent measurements. It is clearly seen that APD-1 has a wedge-shaped E profile in the 300-nm-thick avalanche region and a peak E field near the p-n junction interface. This peak E field exceeds 1100 kV/cm at M=2. APDs 2 and 3 with field control layers show more gentle gradients of E field in the thinner 150-nm-thick avalanche region. Besides, APD-3 has both lower E field gradient and peak E field intensity compared to APD-2 owing to the lower doping density in multiplication layer.
The RT photo and dark current-voltage (I-V) curves for the three APDs with a mesa size of 20×20 μm2 are shown in Fig. 2. It is seen that APDs 1, 2 and 3 show multiplication layers punch through at around −13.7, −13.3 and −10.5 V, respectively, with relatively flat photocurrent responses. The corresponding unity-gain voltage (M=1) was defined to be −13.7, −13.3 and −10.5 V, respectively, by comparing the responsivities with a reference InGaAs p-i-n photodiode with the same absorption layer thickness [14]. M was extracted from photo current (Ip) by scaling the net photo-current (Ip-Id) with that at unity. The deduced corresponding gain-voltage curves are also shown in Fig. 2. The plotted M>1 voltage ranges for APDs 1, 2 and 3 are determined to be 13.7–20.3, 13.3–16.3 and 10.5–21.5 V, respectively, as listed in Table 1. The VBR was determined by defining that the breakdown takes place at a dark current of 10 μA. It is found that APD-2 and APD-3 show the narrowest and the widest operating voltage ranges, respectively, leading to the largest and the least gain-voltage slopes. Such gain performance improvement of APD-3 originates from the reduced doping concentration and thus the attenuated E field gradient in the avalanche region (Fig. 1). For APD-1 with a 300-nm-thick multiplication layer and a relatively high doping density, a moderate operating voltage range as well as gain-voltage slope is observed, which are likely owing to the wedge-shaped E field profile with a high E field gradient. Those results tell that the operating voltage ranges and the gain-voltage slopes of thin multiplication InAlAs/InGaAs APDs can be readily tailored by control of the layer structures, the layer thicknesses and the doping densities in the avalanche regions.
Fig. 2 Solid and dash lines are the RT photo and dark I-V curves, respectively, for APD-1 (black), APD-2 (blue) and APD-3 (red). Mesa size: 20 × 20 μm2. The black open circle, the blue open square and the red open uptriangle indicate the gain-voltage curves for APD-1, APD-2 and APD-3, respectively.
The RT Id of APDs 1, 2 and 3 biased at 0.9 times breakdown voltage (0.9VBR) were determined to be 112, 94, and 128 nA, respectively, as also listed in Table 1. Such relatively high Id of those APDs suggest that the high mean or peak E field intensity in thin multiplication layer APDs will affect their noise performance at high gains. APD-2 shows smaller Id at 0.9VBR than the two other APDs, possibly a combined result of the moderate doping density and the thinner multiplication layer thickness. To uncover the current leakage mechanisms in those thin multiplication layer APDs, temperature dependent Id measurements are carried out. Figure 3 shows the Arrhenius plot of Id for APDs 1, 2 and 3 biased at fixed voltages of −16.5, −14.2 and −15.8 V, respectively, which corresponds to a same gain factor of M=2 for each device at RT. It is seen that Id showed similar weak temperature dependences for APDs 1, 2 and 3 at temperatures below 150, 200 and 175 K, respectively. For higher temperatures, Id of all three APDs showed strong temperature dependencies which vary with 1/T in a linear relation. The activation energies of Id can be fitted by Id∝exp(-Ea/kT), where k is the Boltzmann constant and T is the temperature. The fitted Ea at temperatures of T>250, T>275 and T>225 K for APD-1, APD-2 and APD-3 is 0.19, 0.16 and 0.12 eV, respectively, as indicated in Fig. 3.
Fig. 3 Arrhenius plot of Id for APDs 1, 2 and 3 biased at fixed voltages of −16.5, −14.2 and −15.8 V, respectively. Mesa size: 20×20 μm2. The fitted activation energies at temperatures above 250, 275 and 225 K for APD-1, APD-2 and APD-3, respectively, are indicated. Inset: temperature dependent Id-V curves for APD-1.
Generally, the dominated current leakage mechanism in photodiodes can be qualitatively determined via evaluating the ratio of Ea and the energy bandgap (Eg) of the depletion region material. An Ea of ∼Eg indicates a diffusion component dominated Id, whereas an Ea of ~ Eg/2 suggests a generation-recombination component dominated Id [17–19 ]. Ea in an In0.83Ga0.17As p-i-n photodiode in temperature ranges of T>225 K and 175 K<T<225 K is 0.47 (~Eg) and 0.23 eV (∼Eg/2), respectively, from which Id is ascribed to a diffusion limited process and a generation-recombination limited process, respectively [18]. Ea in InGaAs p-i-n photodiode is ∼0.38 eV at around 260 K, thus Id is dominated by thermal generation through midgap states [19]. However, for the thin multiplication layer APDs in this work, Ea shows much lower values ranging from 0.12 to 0.19 eV at temperatures above 220 K, suggesting different dominating Id mechanisms. Tunneling currents in diodes have weak temperature dependences through the temperature dependence of Eg [20]. The band-to-band tunneling current exponentially depends on E field intensity and becomes substantial when E field intensity exceeds 100 kV/cm [21]. Considering the E field intensities are much larger than 100 kV/cm in the thin multiplication layer APDs in this work, and the E field is confined almost exclusively in avalanche region, it is reasonably suggested that the tunneling currents play the main role in those APDs’ Id. Point defect-induced deep-level traps within the bandgaps of In0.83Ga0.17As [22] and Ge [23] p-i-n photodetectors have been reported with an Ea of ~0.25 eV. Besides, it has been found that with increasing E field intensity such trap-assisted tunneling process are enhanced and Ea shows a decreasing trend [23]. Hence, the tunneling current via defect-induced traps within the forbidden gap of InAlAs multiplication layer is most likely the dominant source Id for those thin multiplication layer APDs at high temperatures (T>220 K). In this temperature range, other thermally activated sources of leakage, e.g. the generation-recombination current or the surface currents, also contribute to the Id. At temperatures lower than 150, 200 and 175 K for APDs 1, 2 and 3, respectively, the Id mainly originated from the band-to-band tunneling currents as well as the surface or edge leakage currents, which have all very weak temperature dependences [24,25]. This accounts for the nearly temperature independent Id characteristics at those temperature ranges. At intermediate temperature ranges, Id is a combined effect of those components.
Inset of Fig. 3 shows temperature dependent Id-V curves for APD-1. A twist caused by the positive temperature coefficient of VBR is clearly observed at around −19 V, which is a signature of avalanche breakdown [11, 14]. The other two APDs show similar temperature dependent Id-V curves.
The temperature dependent spectral responsivity of APDs 1, 2 and 3 were measured using a Fourier transform infrared (FTIR) spectrometer. Results are shown in Fig. 4. Figure 4(a) shows the temperature dependent spectral responsivity of APD-1 biased at a fixed voltage of −13.5 V which is lower than the unity-gain voltage. It is found that the spectral responsivity showed a slight decrease from 77 to 150 K and then a significant increase with increasing temperature. Figures 4(b), 4(c) and 4(d) show the temperature dependent spectral responsivities of APDs 1, 2 and 3, respectively, biased at fixed voltages of −16.5, −14.2 and −15.8 V, respectively, which corresponds to a same gain of M=2 for each device at RT. In contrast to the behavior of APD-1 at M<1, the spectral responsivities showed a monotonic decrease with increasing temperature for all three APDs at M>1. Positive temperature dependent spectral responsivity characteristics similar to Fig. 4(a) have been frequently observed in Au-n+ GaAs Schottky diodes [26], In0.83Ga0.17As p-i-n photodiodes [27] and ZnS or ZnSTe ultraviolet photodiodes [28], and have been interpreted as enhanced minority carrier diffusion length [27] or the change of the density-of-state distribution [28] at high temperatures. However, negative temperature coefficient of spectral responsivity for InGaAs APDs has rarely been reported. Such anomalous temperature dependent behavior is closely related to the avalanche process in the multiplication layer.
Fig. 4 Temperature dependent spectral responsivity of the three APDs with a mesa size of 20×20 μm2. (a) APD-1 biased at −13.5 V (M<1). (b) APD-1, (c) APD-2 and (d) APD-3 biased at −16.5 V, −14.2 V and −15.8 V, respectively, which corresponds to M=2 for each device at RT.
In APDs, the multiplication is essentially an carrier impact ionization process which is closely related to the energy of carriers [9, 10]. For a fixed reverse bias, a nearly constant E field profile is expected since carrier concentration varies only weakly on temperature. With increasing temperature, carriers in the avalanche region suffer increased phonon scattering and thus relax their energy [29], resulting in reduced impact ionization coefficients as well as multiplication currents. Thus the spectral responsivity of APDs shows a negative temperature coefficient at M>1. At M<1, since no avalanche happens, APDs are operated similar to traditional p-i-n detectors with low internal E field intensity. The electron tunneling probability across the band offset at the interface between InGaAs absorber and InAlAs multiplication layer would drop at low temperatures, resulting in attenuated photocurrent. The enhanced minority carrier diffusion length or the change of the density-of-state distribution at high temperatures also possibly contributes to such positive temperature coefficient. Figure 5(a) shows the schematic layer structure of a reference InGaAs p-i-n planner photodiode. The measured temperature dependent spectral responsivity under zero bias is shown in Fig. 5(b). A positive temperature coefficient of the spectral responsivity of the p-i-n detector is clearly observed, which is similar to that of APD-1 at M<1 (Fig. 4(a)). Such similarity proved the reasonability of the above explanation.
Fig. 5 (a) Schematic layer structure of a reference InGaAs p-i-n planner photodiode. (b) Measured temperature dependent spectral response of this detector with a mesa size of 20×20 μm2 at zero bias.
In addition, those temperature dependent Id and spectral responsivity results reported here suggest that the Id and the multiplied photocurrent performances of thin multiplication layer APDs can be greatly improved simultaneously by lowering their operating temperature. Such superior device performances will facilitate the potential applicability of thin multiplication layer APDs in advanced applications, e.g. the Geiger-mode photon counting [30]. It should be pointed out that the bandwidth of those APDs is expected to decrease in a certain degree compared to the reported high-speed InAlAs/InGaAs APDs for communication applications [31], which mainly originates from the moderate doping level within the avalanche regions (1016–1017 cm−3) and thus the undepleted absorber structures. However, the slightly lowered response speed of those devices should still meet the requirements of FPA applications.
For APD-FPA applications, it is highly desirable and feasible to apply a same bias for all pixels, which will drastically reduce the complexity and cost of the ROIC [5]. High pixel gain uniformity is a must in order to realize such a goal. In this work, the gain uniformity of APDs 1, 2 and 3 is quantitatively evaluated by analyzing the statistical distribution of M across the wafers biased at fixed voltages of −18.2, 15.2 and −20.2 V, respectively. Over 50 devices distributed uniformly on an area of 1×1 cm2 were measured. Results are shown in Fig. 6. The distributions for the three APDs can be fitted using a Gaussian function respectively. The fitted mean M (denoted by <M>) of APDs 1, 2 and 3 is 8.2, 7.6 and 7.4, respectively, and the corresponding M dispersion δ (δ equals the ratio of standard deviation and the mean value) is 21.6%, 20.7% and 19.7%, respectively. Those relative small dispersions of M benefit a better compatibility with the Si-based ROIC in FPA applications, and is expected to be corrected in the image acquisition software. Both the excellent epitaxial layer uniformity and the reliable device processing contribute to the achieved high gain uniformity of δ∼20% for the three APDs. Note that APD-1 shows a higher dispersion of M compared to APDs 2 and 3, which is possibly related to the wedge-shaped E field distribution with high E gradients and high E field peak intensities in the avalanche region therein. This also means the heavily-doped charge layer aside the multiplication layer not only helps to control the E field distribution but also helps to improve the gain uniformity via a more uniform E profile. Besides, the slight difference of only 1% in δ between APDs 2 and 3 could be a result of the small E field gradient difference between them (as shown in Fig. 1(b)).
Fig. 6 Statistical M distributions of (a) APD-1, (b) APD-2 and (c) APD-3, biased at fixed voltages of −18.2, 15.2 and −20.2 V, respectively. The fitted Gaussian curves, <M> and M dispersions (δ) are also indicated.
4. Conclusion
In summary, we have demonstrated the tailor of the performance of thin multiplication layer InAlAs/InGaAs SAGCM APDs with operating voltages lower than 20 V. Their operating voltages, gain-voltage slopes and Id were successfully tailored by controlling the E field distributions in avalanche region. The dominant Id sources at T>220 K are identified to be the band-trap-band tunneling while at T<175 K are found to be the band-to-band tunneling and the surface current. The spectral responsivity of those APDs showed anomalous negative temperature coefficients at M>1, which is attributed to the enhanced phonon scattering effect of carriers in the avalanche region at high temperatures. Those APDs show high M uniformity across the wafers at a given bias. The field-control layer is found to help improve the gain uniformity.
Acknowledgments
The authors wish to acknowledge the support of the National Basic Research Program of China under grant No. 2012CB619202, the National Natural Science Foundation of China under grant Nos. 61275113, 61204133 and 61405232 and the Yang Fan Program of Shanghai Municipality under grant No. 15YF1414300.
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