Mohsin Aziz

Deep-level transient spectroscopy of interfacial states in “buffer-free” p-i-n GaSb/GaAs devices

A systematic study was carried out on defect states in Interfacial Misfit (IMF) unpassivated and Te-passivated IMF in p-i-n GaSb/GaAs devices using Deep Level Transient Spectroscopy (DLTS) and Laplace DLTS. Additionally, Current-Voltage (I–V) measurements were performed, which showed that the turn-on voltage (Von) of passivated samples is lower than that for unpassivated samples; an effect which can be explained by the introduction of new defects states near to the interface of GaSb/GaAs, where Te was incorporated to passivate the IMF. The Capacitance-Voltage (C-V) analysis demonstrates that these new states are the consequence of adding Te at the misfit of GaSb/GaAs. Furthermore, DLTS measurements reveal a distribution of states including a main midgap energy level, namely the well documented EL2 trap, with some peculiar behaviour. Most of these levels are related to interface states that are generated by the mismatch between GaAs and GaSb. Originally, the addition of Te atoms was thought to passivate these interface states. On the contrary, this paper, which attempts at correlating the current-voltage and capacitance-voltage characteristics to the DLTS results, shows clearly that Te atoms increase the density of interface states.

Investigation of deep-level defects in conductive polymer on n-type 4H- and 6H-silicon carbide substrates using I-V and deep level transient spectroscopy techniques

The current-voltage (I–V) characteristics of Au/sulfonated polyaniline (SPAN)/n-SiC heterojunctions have been investigated in detail over a wide range of temperatures between 20 and 440 K. The measured I–V characteristics of all devices show a good rectification behavior at all temperatures. The room temperature rectification ratios (forward to reverse currents ratio, IF/IR) at 0.6 V for SPAN/n-type 4H-SiC and SPAN/n-type 6H-SiC heterojunctions are 2 × 104 and 7 × 106, respectively. The value of rectification of SPAN/6H-SiC heterojunction is four orders of magnitutude higher than the state-of-the art sulfonated polyaniline thin films deposited on n-type silicon substrates. A self-assembly technique and copolymerization were used to fabricate a self-doped polyaniline films on SiC substrates. The experimental I–V data were analysed using the Werner model, which includes the series resistance of the heterojunctions. The diode parameters such as the ideality factor and the barrier height are determined from t...

Zinc oxide thin films on silicon carbide substrates (ZnO/SiC): electro-optical properties and electrically active defects

The electrical and optical properties of heterojunctions formed by thermally deposited ZnO thin films on n-type 4H-SiC substrates have been investigated. Current–voltage characteristics of the fabricated light emitting devices revealed excellent rectifying behaviors with a typical leakage current lower than 1 nA at a reverse bias of −3 V, and with a forward current at 3 V in the range of 2 mA. A study of the electroluminescent characteristics of ZnO/SiC heterojunctions over the temperature range of 50–450 K showed an emission peak around 410 nm and a broad defect-related electroluminescence at room temperature in the visible range for a forward current of 300 mA. Electrically active deep level centers in ZnO and n-type 4H-SiC epilayers have been investigated by deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS (LDLTS). Additionally, LDLTS has successfully been employed to resolve the closely spaced hole trap energy levels.

Electrical characterisation of deep level defects in Be-doped AlGaAs grown on (100) and (311)A GaAs substrates by MBE

The growth of high mobility two-dimensional hole gases (2DHGs) using GaAs-GaAlAs heterostructures has been the subject of many investigations. However, despite many efforts hole mobilities in Be-doped structures grown on (100) GaAs substrate remained considerably lower than those obtained by growing on (311)A oriented surface using silicon as p-type dopant. In this study we will report on the properties of hole traps in a set of p-type Be-doped Al0.29Ga0.71As samples grown by molecular beam epitaxy on (100) and (311)A GaAs substrates using deep level transient spectroscopy (DLTS) technique. In addition, the effect of the level of Be-doping concentration on the hole deep traps is investigated. It was observed that with increasing the Be-doping concentration from 1 × 1016 to 1 × 1017 cm-3 the number of detected electrically active defects decreases for samples grown on (311)A substrate, whereas, it increases for (100) orientated samples. The DLTS measurements also reveal that the activation energies of traps detected in (311)A are lower than those in (100). From these findings it is expected that mobilities of 2DHGs in Be-doped GaAs-GaAlAs devices grown on (311)A should be higher than those on (100).

Multispectral mid-infrared light emitting diodes on a GaAs substrate

We have designed, simulated, and experimentally demonstrated four-colour mid-infrared (mid-IR) Light Emitting Diodes (LEDs) integrated monolithically into a vertical structure on a semi-insulating GaAs substrate. In order to finely control the peak wavelength of the emitted mid-IR light, quantum well (QW) structures based on AlInSb/InSb/AlInSb are employed. The completed device structure consists of three p-QW-n diodes with different well widths stacked on top of one bulk AlInSb p-i-n diode. The epitaxial layers comprising the device are designed in such a way that one contact layer is shared between two LEDs. The design of the heterostructure realising the multispectral LEDs was aided by numerical modelling, and good agreement is observed between the simulated and experimental results. Electro-Luminescence measurements, carried out at room temperature, confirm that the emission of each LED peaks at a different wavelength. Peak wavelengths of 3.40 μm, 3.50 μm, 3.95 μm, and 4.18 μm are observed in the bulk...

Investigation of electrically active defects in InGaAs quantum wire intermediate-band solar cells using deep-level transient spectroscopy technique

InGaAs quantum wire (QWr) intermediate-band solar cell-based nanostructures grown by molecular beam epitaxy are studied. The electrical and interface properties of these solar cell devices, as determined by current-voltage (I-V) and capacitance-voltage (C-V) techniques, were found to change with temperature over a wide range of 20-340 K. The electron and hole traps present in these devices have been investigated using deep-level transient spectroscopy (DLTS). The DLTS results showed that the traps detected in the QWr-doped devices are directly or indirectly related to the insertion of the Si δ-layer used to dope the wires. In addition, in the QWr-doped devices, the decrease of the solar conversion efficiencies at low temperatures and the associated decrease of the integrated external quantum efficiency through InGaAs could be attributed to detected traps E1QWR_D, E2QWR_D, and E3QWR_D with activation energies of 0.0037, 0.0053, and 0.041 eV, respectively.

A new monolithic approach for mid-IR focal plane arrays

Antimonide-based photodetectors have recently been grown on a GaAs substrate by molecular beam epitaxy (MBE) and reported to have comparable performance to the devices grown on more expensive InSb and GaSb substrates. We demonstrated that GaAs, in addition to providing a cost saving substrate for antimonide-based semiconductor growth, can be used as a functional material to fabricate transistors and realize addressing circuits for the heterogeneously grown photodetectors. Based on co-integration of a GaAs MESFET with an InSb photodiode, we recently reported the first demonstration of a switchable and mid-IR sensible photo-pixel on a GaAs substrate that is suitable for large-scale integration into a focal plane array. In this work we report on the fabrication steps that we had to develop to deliver the integrated photo-pixel. Various highly controllable etch processes, both wet and dry etch based, were established for distinct material layers. Moreover, in order to avoid thermally-induced damage to the InSb detectors, a low temperature annealed Ohmic contact was used, and the processing temperature never exceeded 180 °C. Furthermore, since there is a considerable etch step (> 6 μm) that metal must straddle in order to interconnect the fabricated devices, we developed an intermediate step using polyimide to provide a smoothing section between the lower MESFET and upper photodiode regions of the device. This heterogeneous technology creates great potential to realize a new type of monolithic focal plane array of addressable pixels for imaging in the medium wavelength infrared range without the need for flip-chip bonding to a CMOS readout chip.

Electrical Behavior of MBE Grown Interfacial Misfit GaSb/GaAs Heterostructures With and Without Te-Doped Interfaces

A detailed study of interface states in interfacial misfit (IMF) grown GaSb on GaAs substrates is presented. Two types of structures, namely, uncompensated and Te compensated, are investigated using current-voltage, capacitance-frequency, conductance-frequency, and deep level transient spectroscopy techniques. Our studies reveal that incorporation of Te at the interface (IMF) causes a degradation of the Te-compensated devices. A higher number of electrical active defects and higher value of interface states are detected in Te-compensated IMF GaSb/GaAs devices compared with as-grown IMF GaSb/GaAs devices.

Monolithic fabrication of InSb-based photo-pixel for Mid-IR imaging

We present the monolithic fabrication of an active photo-pixel made in InSb on a GaAs substrate that is suitable for large-scale integration into a focal plane array. Pixel addressing is provided by the co-integration of a GaAs MESFET with an InSb photodiode. Various highly selective etch processes for distinct material layers were established. A low temperature annealed Ohmic contact was also achieved so that the processing temperature never exceeded 180 °C and no damage to the InSb detectors was observed. Furthermore, since there is a considerable etch step (> 6 μ) that metal must straddle without breaking, we developed an intermediate step using polyimide to provide a smoothing section between the lower MESFET and upper photodiode regions of the device. This heterogeneous technology creates great potential to realize a new type of monolithic focal plane array of addressable pixels for imaging in the medium wavelength infrared range.