Neha Aggarwal

Fabrication of non-polar GaN based highly responsive and fast UV photodetector

We report the fabrication of ultraviolet photodetector on non-polar (11–20), nearly stress free, Gallium Nitride (GaN) film epitaxially grown on r-plane (1–102) sapphire substrate. High crystalline film leads to the formation of two faceted triangular islands like structures on the surface. The fabricated GaN ultraviolet photodetector exhibited a high responsivity of 340u2009mA/W at 5u2009V bias at room temperature which is the best performance reported for a-GaN/r-sapphire films. A detectivity of 1.24u2009×u2009109 Jones and noise equivalent power of 2.4u2009×u200910−11 WHz−1/2 were also attained. The rise time and decay time of 280u2009ms and 450u2009ms have been calculated, respectively, which were the fastest response times reported for non-polar GaN ultraviolet photodetector. Such high performance devices substantiate that non-polar GaN can serve as an excellent photoconductive material for ultraviolet photodetector based applications.

Probing the correlation between structure, carrier dynamics and defect states of epitaxial GaN film on (110) sapphire grown by rf-molecular beam epitaxy

A systematic study has been performed to correlate structural, optical and electrical properties with defect states in the GaN films grown on a-plane (110) sapphire substrate via rf-plasma molecular beam epitaxy. Morphological analysis reveals the presence of small lateral size (30–70 nm) hexagonally shaped V-pits on the GaN films. These V-defects possibly contribute as the main source of non-radiative decay. High resolution X-ray diffraction reveals highly single crystalline GaN film grown on a-plane sapphire substrate where the threading dislocations are the cause of V-defects in the film. Photoluminescence measurement shows a highly luminescence band to band emission of GaN film at 3.41 eV along with a broad defect band emission centered at 2.2 eV. A detailed optical and electrical analysis has been carried out to study the defect states and related carrier dynamics for determining the efficacy of the film for device fabrication. The variation in the low temperature current voltage measurements confirms the presence of deep level defects in the mid-band gap region while transient spectroscopy shows that non radiative decay is the dominant relaxation mechanism for the photo excited-carriers from these defect states.

Correlation of current–voltage–temperature analysis with deep level defects in epitaxial GaN films

The effect of temperature on the nature of metal-semiconductor system in a Au contact deposited on c-plane and a-plane GaN film was investigated by current–voltage (I–V) measurements. The I–V measurements have been obtained systematically at different temperatures ranging from room temperature (300u2009K) to low temperature (78u2009K). Photoluminescence measurements were obtained to investigate correlation between the growth conditions, the substrate used for the growth of GaN film, and the presence of deep level defects therein by equating with the yellow band luminescence. The resistance–voltage–temperature analysis indicates that a gradual shift of the nature of contact towards Schottky behavior takes place while moving from room temperature to low temperature. Additionally, memory effect like aberration is present at low temperature, which can be attributed to the presence of deep-level defects and carrier recombination therein.

Carrier relaxation dynamics in defect states of epitaxial GaN/AlN/Si using ultrafast transient absorption spectroscopy

The relaxation dynamics of the carriers through the defect levels in an epitaxial GaN film grown with an AlN buffer layer on Si has been performed on the femto–picosecond timescale, using ultrafast transient absorption spectroscopy (UFTS). The sample was pumped above and below the band gap and probed with a white light continuum (480–800 nm). A combination of bi and triple exponential decay functions at different probe wavelengths were used to fit the kinetic profile of the carriers in the defect continuum. Based on the UFTS measurements, a model is proposed which explains the dynamics in the shallow traps and deep level defects. Furthermore, to determine the role of the lattice in the relaxation dynamics, the experiment was conducted at a low lattice temperature of 4.2 K. The relaxation constants from the UFTS measurements confirm not only the presence of shallow and deep level defects but also the involvement of phonons in one of the relaxation processes.

GaN-UV photodetector integrated with asymmetric metal semiconductor metal structure for enhanced responsivity

Fabrication of very thin GaN ultraviolet photodetectors on Si (111) substrate integrated with asymmetric (Pt–Ag, Pt–Cr) metal–semiconductor–metal (MSM) structure have been illustrated. Designed GaN photodetection device displays significant enhancement in responsivity for asymmetric (Pt–Ag) MSM structure (280xa0mA/W) in comparison to symmetric (Pt–Pt) MSM structure (126xa0mA/W) at 10xa0V bias. The fabricated asymmetric and symmetric devices also exhibit fast response time in the range of 30–59xa0ms. The enhancement in responsivity using asymmetric MSM structure ascribed to large difference in work function which lead to change in Schottky barrier height of the metal semiconductor junction. Additionally, power dependent photoresponse analysis of GaN asymmetric (Pt–Ag) ultraviolet photodetector was showing a responsivity of 116xa0mA/W at low optical power of 1xa0mW. Such GaN asymmetricxa0MSM ultraviolet photodetectors having high responsivity can extensively be used for low power, high speed ultraviolet photo detection applications.

Effect of Metal Contacts on a GaN/Sapphire-Based MSM Ultraviolet Photodetector

We report the fabrication of a GaN/sapphire ultraviolet (UV) photodetector (PD) with asymmetric (Pt-Ag) and symmetric (Pt-Pt) metal–semiconductor–metal (MSM) structure. The fabricated devices display Schottky behavior. Time-dependent photoresponse analysis reveals a significant enhancement in photocurrent leading to a photoresponsivity of 37xa0mA/W and 267xa0mA/W under 13xa0mW optical power at 5xa0V bias for Pt-Pt and Pt-Ag devices, respectively. Further, power-dependent measurements reveal a peak responsivity of 633xa0mA/W at 4mW optical power and 5xa0V bias for the Pt-Ag asymmetric MSM photodetector. The significant enhancement in responsivity for asymmetric MSM UV PD is ascribed to the built-in potential gradient in the GaN semiconductor, which drives a large number of charge carriers for enhanced charge collection. Further, the noise equivalent power was lowest for the Pt-Ag MSM structure, which was calculated to be 4.6u2009×u200910−14xa0WHz−1/2. This high performance asymmetric MSM GaN UV PD can be integrated into efficient UV PD-based applications.