Electrical, Optical and High Frequency Performance Improvement for Type-II Superlattices Based Photodetectors

Li, Jiakai

doi:https://doi.org/10.21985/n2-9vga-v489

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Electrical, Optical and High Frequency Performance Improvement for Type-II Superlattices Based Photodetectors

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Since Infrared radiation was discovered in 1800s, the research and applications on the infrared regime have been continually developed. The infrared detectors are the key technology in these applications and have been successfully used for medical imaging, light detection and ranging (LiDAR), free-space optical communication, target tracking and object identification and night vision, etc. Due to the rapid progress of solid-state physics and devices since 1950s, the infrared photon detectors have seen significant development and shown advantages like high sensitivity and short signal response time over the old generation of infrared detectors, thermal detectors. Commercial infrared photodetectors are currently dominated by HgCdTe or compound semiconductor bulk materials based p-i-n photodiodes, which are facing many limitations such as non-uniformity, material growth difficulties, high capacitance and lack of internal gain. Recent advances in Ⅲ-Ⅴ semiconductor materials growth and processing led to the demonstration of novel quantum structures such as Type-Ⅱ superlattices (T2SLs). The advantages of T2SLs include good uniformity, great band structure engineering tunability, auger recombination suppression and wide range cut-off wavelength coverage, which make T2SLs a promising candidate for infrared detection and imaging from short wavelength infrared to long wavelength infrared. Until now, most of the developed T2SLs-based infrared photodetectors are p-i-n structure photodiodes or different types of barrier photodetectors such as nBn structure, which lack the internal gain mechanism therefore cannot be used in some applications that requires detection of weak photon signals such as LIDAR, optical telecommunications, and quantum computing.The extensively used gain-based photodetectors at present include phototransistors and avalanche photodiodes. The goal of this PhD thesis is to explore the possibilities of applying the Type-Ⅱ superlattices into the heterojunction phototransistors and avalanche photodiodes, investigate the design, growth and fabrication of these devices based on T2SLs to achieve better optical, electrical, and high-speed performance over the currently existing devices. An HPT is the integration of a photodiode and a bipolar junction transistor (BJT) into one device. The BJT will functionally amplify the photo-generated signal from the photodiode. A two-terminal npn HPT device consists of n-type doped wide-bandgap emitter, p-type doped base and n-type doped collector. Different kinds of Type-II superlattices were carefully chosen for the emitter, base, and collector to improve the optical performance with the help of the Empirical Tight Binding method (ETBM). The molecular beam epitaxy (MBE) growth of the HPT devices was continually improved to achieve successfully high material quality which was mainly characterized by X-ray diffraction (XRD) and atomic force microscope (AFM). The effects of different device parameters include emitter doping concentration, base doping concentration, base thickness and energy bandgap difference between emitter and base on the dark current and optical gain of the HPTs have been investigated. By scaling the base thickness, the optical gain of 345.3 at 1.6 μm at room temperature has been achieved for T2SLs-based HPTs. The HPT devices can also be an alternative technology that can demonstrate high speed in the optical telecommunications with high system transmission rates since the structure of HPT is similar to that of heterojunction bipolar transistors (HBTs); and as such, HPTs can be well suited for high-speed performance. High speed performance of heterojunction phototransistors (HPTs) based on InAs/GaSb/AlSb type-II superlattice with 50% cut-off wavelength of 2.0μm at room temperature were designed, grown, fabricated, and measured by the newly developed measurement system. The relationship between −3 dB cut-off frequency of these HPT devices versus mesa size, applied bias, collector layer thickness, and measurement temperature were investigated to optimize the device designs. By tuning the device structure like scaling the diode size or collector thickness, the -3 dB cut-off frequency of 2.8 GHz and 5.1 GHz at 300 K and 150 K has been demonstrated for the SWIR T2SLs-based HPTs, respectively. The trade-off between photo response and high-speed performance in terms of the absorption/collector layer thickness of the T2SLs-based HPTs was also observed and analyzed. In order to break the trade-off between the optical performance and high-speed performance, the active layer of T2SLs-based HPT was placed in a designed resonant cavity structure to offer multiple passes for the incident light thus giving more possibility for the light to be efficiently absorbed. The resonant cavity structure has been widely applied in the commercial optoelectronic devices such as light-emitting diode and vertical-cavity surface-emitting laser. At the range of short-wavelength infrared, the resonant cavity enhanced (RCE) structures have been incorporated into GaAs, InP and SiGe based photodetectors. In this work, resonant cavity enhanced heterojunction phototransistor (RCE-HPT) based on InAs/GaSb/AlSb T2SLs grown by MBE has been demonstrated. An eleven-pair lattice matched GaSb/AlAsSb quarter-wavelength Bragg reflector was used as the bottom mirror in the RCE-HPT structure. The T2SLs-based RCE-HPT device with relatively thin absorption layer exhibited advantages like lower dark current, the wavelength selectivity and a cavity enhancement of optical responsivity for short wavelength infrared detection at room temperature over our previous common T2SLs-based HPTs. Except for heterojunction phototransistors, avalanche photodiodes (APDs) are another kind of extensively used gain-based photodetectors. APDs can deliver high sensitivity involved with gain mechanism via avalanche multi-plication with several applications in military and fiber-optic communication, imaging and commercial sector. According to the local-field avalanche theory, the excess noise factor of an APD is determined by the k factor which is the ratio of the hole (β) and electron (α) ionization coefficients of the APD. As demonstrated by McIntyre, a large difference in the ionization rates for electrons and holes (low k factor) is essential for a low noise avalanche photodiode. 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" de-creasing k in these materials by using APDs with band structure-engineered avalanche regions. One of the possible alternatives of impact ionization engineering for APDs is by using the multi-quantum well (MQW) structure as the avalanche region. The flexibility of Sb-based strained layer superlattices band structure engineering has a significant advantage for designing multi quantum well (MQW)-based APD. In this MQW structure the band discontinuities between well and barrier can be engineered to have a large conduction band discontinuity and a small valence band discontinuity. In the MQW structure, electron ionization rate can be enhanced since the electrons receive kinetic energy at hetero interfaces. Holes, on the other hand, can flow unhindered across the MQW because almost vanishes. Therefore, I designed an MWIR APD with the multiplication layer consisting of an MQW structure, which consists of AlAsSb/GaSb superlattice as the barrier layer and InAsSb as the well layer. The SAM-APD device was designed to have electron dominated avalanche mechanism via the band structure engineered multi-quantum well structure. The device exhibits a maximum multiplication gain of 29 at 200 K under -14.7 bias voltage. The electron and hole impact ionization coefficients were derived and the large difference between their value was observed. The carrier ionization ratio for the MWIR SAM-APD device was calculated to be ~0.097 at 200 K. To improve photodetectors’ performance, one way is to increase the optical response such as using the gain-based photodetectors. Another direction is to reduce the dark current density of the photodetectors. Current T2SL photodetectors are based on a fully etched mesa-delineated pixel configuration, where plasma or chemical etching is used to delineate and isolate the devices. A critical step in the processing of mesa devices is the etching, followed by the surface treatment and deposition of a dielectric passivant on the exposed junction in order to reduce surface currents. In contrast, device processing of planar structures has less requirement for surface passivation because the junction interface is buried. Planar photodiodes should further reduce manufacturing costs, through simpler device processing and higher yield, while reducing surface leakage currents and increasing the operating temperature compared to a mesa etched device. The planar T2SL device design has several key benefits over traditional mesa isolated designs. By moving to a planar geometry, the exposed mesa sidewalls are eliminated completely. This prevents sidewall contamination from increasing the dark current of the device, and as such it is no longer necessary to passivate the device. This greatly simplifies the processing of Type-II photodetectors and imagers since one of the main challenges to realizing high-performance small area devices is developing effective etching, cleaning, and passivation. A mid-wavelength infrared planar photodetector based on InAs/InAs1-xSbx type-Ⅱ superlattice using Zinc diffusion was then fabricated using our newly developed planar T2SL device processing method. The active layer for the photodetector was first grown by a Solid Source Molecular Beam Epitaxy reactor to achieve high quality material. Then the samples were sent to a EMCORE metal-organic chemical vapor deposition (MOCVD) reactor to perform Zn diffusion. At 150 K, the MWIR planar photodetector showed a dark current density more than two orders of magnitude lower than the mesa-isolated device and an improved detectivity of 2.0×1011 cm˙Hz1/2/W at 3.7 μm.

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