Short wavelength infrared avalanche photodetector using Sb-based strained layer superlattice

We demonstrate a low noise short wavelength infrared (SWIR) Sb based type II superlattice (T2SL) avalanche photodiodes (APD). The SWIR GaSb/(AlAsSb/GaSb) APD structure was designed based on impact ionization engineering and grown by molecular beam epitaxy on GaSb substrate. At room temperature, the device exhibits a 50 % cut-off wavelength of 1.74 micron. The device revealed to have electron dominated avalanching mechanism with a gain value of 48 at room temperature. The electron and hole impact ionization coefficients were calculated and compared to give better prospect of the performance of the device. Low excess noise, as characterized by the carrier ionization ratio of ~ 0.07, has been achieved.

Avalanche photodiodes (APDs) internally amplify charge carriers with an avalanche process while operating under a high reverse bias that can cause impact ionization compared to conventional p-i-n photodiodes. APDs can deliver high sensitivity involved with gain mechanism via avalanche multiplication with several applications in military and fiber-optic communication, imaging and commercial sector. [1][2][3][4][5] For short-wavelength infrared (SWIR) APDs, several material systems are implemented, including silicon, AlGaAs/InGaAsSb, InP/InGaAs, and HgCdTe (MCT) [6][7][8]. However, the spectral band between 1.5-2.6 µm of the SWIR range can be served further compared to InP/InGaAs or MCT. Lattice matched InGaAs/InP can deliver high performance APD devices operating in the 0.9 to 1.7 µm wavelength range. InGaAs detectors are capable to reach longer cut-off wavelengths by increasing the indium content, however, the crystal defects introduced by the epitaxial process used to extend the indium content degrade the performance as the cut-off wavelength gets longer. MCT on the other hand is the most mature material system for infrared a) Corresponding author: razeghi@eecs.northwestern.edu technology, but it suffers from drawbacks due to bulk and surface instability and higher costs particularly for fabrication. [9,10] Due to the nature of impact ionization the avalanche process is a random process, which is associated to a factor named excess noise F(M). According to the local-field avalanche theory, both the F(M) and the gain-bandwidth product of an APD can be impacted by the k factor which is the ratio of the hole (β) and electron (α) ionization coefficients of the APD. As demonstrated by McIntyre [11] a large difference in the ionization rates for electrons and holes (low k factor) is essential for a low noise avalanche photodiode. The F(M) which is given by: rises with increasing the gain (M), but the rate of increasing noise can be slow down by reducing the k value. Lower k value can improve the performance of the APD devices. [12,13] Therefore low k factor is crucial for high-speed and low-noise operation of APD device. The k value can be minimized under single-carrier-initiated single-carrier multiplication (SCISCM) conditions (means that an APD must be operated such that only one carrier species ionizes). [14,15] This is difficult when for some materials the impact ionization coefficients are similar (β/α = k ≅ 1); it is therefore of great interest to explore the possibility of "artificially" decreasing k in these materials by using APDs with bandstructure-engineered avalanche regions.
There is a great opportunity for introducing a new material capable of low dark current, high quantum efficiency, and single carrier multiplication for use in strategic SWIR range. Antimony (Sb)-based III/V materials (bulk and superlattice) are capable of meeting the bandgap requirements for making APDs in the SWIR spectral range. In order to achieve great characteristics for Sb-based APD with high gain and low noise, the bandgap and its electron and hole ionization coefficients have to be designed carefully. To minimize the excess noise factor, a pure or dominant electron or hole initiated multiplication along with optimized hetero-junction design can be applied via impact ionization engineering. [16] One of the possible alternative of impact ionization engineering for SWIR APDs is by using the multi-quantum well (MQW) structure as the avalanche region. In MQW-based avalanche region, the impact ionization happens easily between the heterointerfaces between the barrier and well layers due to a sharp bandgap discontinuities. [17] Sb-based strained layer superlattice (SLS) material is [18] a developing material system with flexible band gap engineering and capabilities to cover the entire range of infrared light using different combination and compositions of Sb based heterostructures, such as InAs/GaSb/AlSb or InAs/InAsSb with Type II staggered gap (type II) band alignment [19][20][21][22]. Recently new gainbased structures including APDs based on SLS Sb-base material have also been reported for SWIR region. [23][24][25][26] The flexibility of T2SLs band structure engineering has a significant advantage for designing multi quantum well (MQW)-based APD [27]. In this MQW structure the band discontinuities between well and barrier can be engineered to have a large conduction band discontinuity (ΔEc) and a small valence band discontinuity (ΔEv). In the MQW structure, electron ionization rate can be enhanced, since the electrons receive kinetic energy ΔEc at hetero interfaces. Holes, on the other hand, can flow unhindered across the MQW because ΔEv almost vanishes In this letter, we demonstrate a SWIR APD structure based on MQW structure consisting of 40 loops of bulk GaSb well layer and AlAsSb/GaSb T2SL structure barrier layer sandwiched between two highly doped contact layers. The schematic of the design and structure of the SWIR APD device is shown in Fig 1a. The device structure was grown on 2-inch Te-doped n-type (10 17 cm -3 ) GaSb (100) substrate using an Intevac Modular Gen II molecular beam epitaxy (MBE) system. As first step 100 nm thick GaSb buffer layer was grown. Then, a 500 nm thick n-contact GaSb layer was grown. During growth, silicon and beryllium were used for n-type and p-type dopant, respectively. The empirical tight-binding method (ETBM) with sps* formalism, with nearest neighbor interactions, under a two-center approximation, which was modified from previous work [28] was used to calculate the band discontinuities in absorption region between the AlAsSb/GaSb superlattice barrier (55 nm) and the GaSb (45 nm) well. Both barrier and well region were left undoped. The ETBM material parameter sets in the previous work were used [28]. The ∆Ec and ∆Ev between the barrier and well in the MQW structure were calculated to be ~0.50 eV and ~0.15 eV, respectively.
Energy-band diagram of the GaSb/(AlAsSb/GaSb) superlattice structure (unbiased and under bias voltage are schematically illustrated in Fig 1(b), (c). In this MQW structure, consider a hot electron accelerating in an AlAsSb/GaSb barrier layer under the bias voltage applied to the structure. When it enters in the GaSb well, it abruptly gains energy equal to the conduction band offset edge (∆Ec). The main effect is that the electron goes under a stronger electric field (increased by ∆Ec). In contrast, the hole ionization rate is not substantially increased by the superlattice because the valence-band discontinuity is much smaller, which leads to have a reduction in the k value. After the MBE growth, the material quality of the SWIR APD sample was assessed using atomic force microscopy (AFM) and high-resolution X-ray diffraction (HR-XRD).
In order to verify the cut-off wavelength of the SWIR APD devices, they were optically characterized using a temperature and pressure-controlled Janis STVP-100 two chamber liquid helium cryostat station with 300 K background. The optical response of the SWIR APD was done under front-side illumination at room temperature. No anti-reflection coating was applied to the devices. The photodetector spectral response was measured using a Bruker IFS 66v/S Fourier transform infrared spectrometer (FTIR) and the absolute responsivity of the device was calculated using a band-pass filter in front of the calibrated blackbody source at 1000 °C.  avalanche mechanism, as seen in similar SWIR APDs. At 300 K, the device shows a unity optical gain dark current of 3.66 × 10 -6 A at -19 V applied bias. The diodes show punch-through effect at the voltage near ∼ -19 V. This effect is considered to be related to the voltage required to achieve a full depletion of the absorption layer, however, more future study is needed to confirm this hypothesis for present work.
We illuminated the device from either top p+ contact and top n+ contact to control the dominate carrier injection into the multiplication region. [29][30][31] For top contact illumination of n+ contact, a separate device with flipped structure was grown and processed under the same condition. In general, the electron and hole impact ionization coefficients, α and β can be derived from the experimental value of electron initiated avalanche gain and hole initiated avalanche gain (Me and Mh) by solving the avalanche rate equations [32]. The extracted electron and hole impact ionization rate for SWIR APD is shown in  For top contact illumination of n+ contact, a separate device with flipped structure was grown and processed under the same condition. The large difference in α and β (see Figure 4) under these different injection regimes is direct evidence that the effective α is greater than β. It also implies that avalanche multiplication process is dominated by impact ionization of electrons. The impact ionization coefficient for the SWIR APD was calculated to be 0.07 = k at room temperature (300K). This small k ratio is largely due to enhanced electron impact ionization, which also agrees fairly well with theoretical predictions and experimental results of this effect in a superlattice with characteristics similar to ours. [17,27] The gain of the device was measured at different temperature as illustrated in   the range 0.2 and 0.3). [16,33,35] By further development of the device architecture and implementing band engineering in the superlattice system, low noise based SWIR device based on SLS material is achievable for low noise application with higher gain bandwidth product.
In summary, using impact ionization engineering Sb-based SLS material structure was implemented to demonstrate a low noise SWIR GaSb/(AlAsSb/GaSb) superlattice APD device.
The multiplication gain of 48 was achieved at room temperature. The structure was designed based on impact ionization engineering by implementing the MQW structure. The device exhibits a 50 % cut-off wavelength of 1.74 µm at room temperature. The electron and hole impact ionization coefficients for the SWIR APD device was calculated and compared with each other to give better prospect of the performance. This leads to extracting the carrier ionization ratio with the value of 0.07 for the SWIR APD. The SWIR APDs revealed promising gain/noise characteristics for low noise applications.