J. S. Ng

Field dependence of impact ionization coefficients in In/sub 0.53/Ga/sub 0.47/As

Electron and hole ionization coefficients in In/sub 0.53/Ga/sub 0.47/As are deduced from mixed carrier avalanche photomultiplication measurements on a series of p-i-n diode layers, eliminating other effects that can lead to an increase in photocurrent with reverse bias. Low field ionization is observed for electrons but not for holes, resulting in a larger ratio of ionization coefficients, even at moderately high electric fields than previously reported. The measured ionization coefficients are marginally lower than those of GaAs for fields above 250 kVcm/sup -1/, supporting reports of slightly higher avalanche breakdown voltages in In/sub 0.53/Ga/sub 0.47/As than in GaAs p-i-n diodes.

Photoluminescence beyond 1.5 μm from InAs quantum dots

Photoluminescence measurements were carried out to investigate the origin of long wavelength emissions (~1.6@mm at room temperature) observed from wafers with InAs quantum dots capped with GaAsSb layers. For wafers with high Sb content (22% and 26%) photoluminescence peak energies were found to be linearly proportional to third root of optical excitation power, a characteristic of emission due to a type-II band alignment. This work therefore presents unambiguous evidence that the long wavelength emission of the wafers comes from type-II band alignment between the InAs quantum dots and the GaAsSb capping layers.

Avalanche Breakdown Characteristics of Al 1– x Ga x As 0.56 Sb 0.44 Quaternary Alloys

Avalanche breakdown characteristics are essential for designing avalanche photodiodes. In this letter, we investigated the effects of adding Ga to Al_1-xGa_xAs_0.56Sb_0.44 quaternary alloys. Using p-i-n diodes with a 100-nm i-region and alloy composition ranging from x=0 to 0.15, we found that the bandgap energy of Al_1-xGa_xAs_0.56Sb_0.44 reduces from 1.64 to 1.56 eV. The corresponding avalanche breakdown voltage decreases from 13.02 to 12.05 V, giving a reduction of 64.7 mV for every percent addition of Ga. The surface leakage current was also found to be significantly lower in the diodes with x=0.10 and 0.15 suggesting that Ga can be added to reduce the surface leakage current while still preserving the lattice match to InP substrate. The data from this letter can be downloaded freely.

Photoluminescence beyond 1.5μm from InAs quantum dots

Photoluminescence measurements were carried out to investigate the origin of long wavelength emissions (~1.6@mm at room temperature) observed from wafers with InAs quantum dots capped with GaAsSb layers. For wafers with high Sb content (22% and 26%) photoluminescence peak energies were found to be linearly proportional to third root of optical excitation power, a characteristic of emission due to a type-II band alignment. This work therefore presents unambiguous evidence that the long wavelength emission of the wafers comes from type-II band alignment between the InAs quantum dots and the GaAsSb capping layers.

Avalanche photodiodes beyond 1.65 μm

There are many applications where the ability to detect optical signals in the 1.65 - 3 μm wavelength range would be of considerable interest. In this paper we discuss two technologies that offer considerable promise for high speed, high sensitivity detection in this region utilising avalanche gain. InGaAs/GaAsSb Type II superlattices as the absorption region and InAlAs as the multiplication region can be combined to form a separate absorption and multiplication (SAM) avalanche photodiode (APD), all grown lattice matched on InP substrates. Detection at room temperature up to 2.4 μm can be readily achieved as can gains in excess of 40. InAs homojunction p-i-n diodes are capable of detecting light with wavelengths > 3 μm, even at 77 K. Although controlling the surface leakage current is a major challenge in mesa devices of InAs, gains in excess of 40 have also been obtained in these devices at room temperature. InAs is also the only III-V semiconductor material that appears to show excess noise-free avalanche gain when electrons are used to initiate the avalanche multiplication. We will discuss recent developments in these two material systems to date and the current state of the technology.

Temperature dependence of avalanche gain in Al 0.85 Ga 0.15 As 0.56 Sb 0.44 APD

We investigated the temperature dependence of dark current, avalanche gain and breakdown voltage in Al0.85Ga0.15As0.56Sb0.44 (hereafter AlGaAsSb) avalanche photodiodes (APDs) with avalanche region widths, w = 90 and 178 nm. There is negligible band to band tunnelling currents and a very weak temperature dependence of gain observed in both didoes. The temperature coefficient of breakdown voltage, Cbd = 0.41 mV/K in AlGaAsSb diode with w = 90 nm is the lowest value reported for different semiconductor materials with similar avalanche region width.

InGaAsN as absorber in APDs for 1.3 micron wavelength applications

Two issues with using InGaAsN as absorber in avalanche photodiodes (APDs) for 1310nm wavelength applications are addressed here. Firstly, we demonstrated InGaAsN p-i-n diodes with stable photoresponse around 1310nm but reverse leakage current density slightly above the acceptable limit of ~0.2mA/cm² at 150kV/cm. We also investigated whether or not InGaAsN as absorber is compatible with Al<inf>0.8</inf>Ga<inf>0.2</inf>As (the proposed avalanche material in our separate-absorption-multiplication APD design) in terms of the relationship between α and β in InGaAsN. Our observations suggest α ~ β in InGaAsN, making it compatible with Al<inf>0.8</inf>Ga<inf>0.2</inf>As.

Type-II photodiode and APD for detection in the 2–2.5 µm wavelength range

The authors report the avalanche characteristics of the type-II absorption region when subjected to electric fields ranging from 60 kV/cm to 260 kV/cm and compare its behaviour to that of bulk In0.53Ga0.47As. Electron and hole initiated multiplication is performed on the type-II PIN photodiodes with 150 pairs of In0.53Ga0.47As/GaAs0.51Sb0.49 layers in the intrinsic region and highly doped In0.53Ga0.47As claddings. The electron to hole ionization ratio is found to be large, and the ionization behaviour of this type-II material system is found to be fairly similar to that of In0.53Ga0.47As. SACM avalanche photodiodes (APDs) utilising the type-II absorption layer is also reported with InAlAs as the multiplication layer with a cut off wavelength of 2.5 mum. The device exhibited low dark current densities at punchthrough of 5.5 mAcm-2 at room temperature and nearly 3 orders of magnitude lower dark currents at 200 K. The breakdown voltage temperature dependence is 40 m V/K, which is lower compared to that of a SACM APD with InP as the multiplication layer. The device also exhibited gains >100 at 200 K.

Dark current mechanisms in In x Ga 1-x As 1-y N y

In order to extend the photo response of GaAs to optical telecommunication wavelengths, In and N can be incorporated into GaAs to yield a perfect lattice match of In_xGa_1-xAs_1-yN_y with GaAs with a bandgap that strongly decreases with increasing N composition. The potential usage of such a material as photodetectors and photovoltaic applications has been reported.In this work, we investigate the dark current mechanisms in the In_xGa_1-xAs_1-yN_y material.

Dark current mechanisms in bulk GaInNAs photodiodes

We have grown a series of bulk GaInNAs p-i-n diodes and identified some of the dark current mechanisms present in our devices. With a nitrogen composition of ~4 %, the band gap can be reduced to 0.94 eV. We also demonstrate that low dark current density is achievable without compromising the absorption and hence quantum efficiency up to 1.4 mum.