Low Temperature Photoluminescence, Transient Photoconductivity and Microwave Reflection for Optical Properties and Transport in PLD‐GaN

Abstract

Optical and transport data on GaN samples grown by low-temperature pulsed laser deposition are presented. Large below-gap band tails are observed in optical absorption spectroscopy. The most intense photoluminescence lines of medium crystalline quality samples can be attributed to excitons bounded to stacking faults. The samples of the highest quality show, besides the ubiquitous yellow band, a large near band emission (NBE) peaking at 3.47 eV. The large width of the NBE is attributed to the joint effects of the band tails and an elevated carrier concentration. Secondary photocurrent transients show a slow decay reaching the millisecond time region for a variety of excitation wavelengths and intensities, with a power law decay exponent of around --0.3. The long times are attributed to the trapping of excess carriers in extended band tail states. From the comparison of photocurrent decay with excess carrier-induced microwave reflection it is concluded that the decay dynamics at the air/film and film/substrate interfaces are identical.Introduction GaN is one of the base materials for modern electronic and optoelectronic devices such as light emitting diodes (LEDs), solar blind UV detectors, laser diodes (LDs) or high-mobility electron transistors (HEMTs) [1]. Despite impressive technical advances, some basic materials issues remain to be resolved. One of these is the elevated density of unwanted impurities that appear in the material independently of the deposition technique. The metalorganic chemical vapour deposition (MOCVD) technique, for instance, relies always on a precursor gas that may introduce unwanted effects.We presented recently a new approach for depositing GaN films, a hybrid, two-step, low-temperature pulsed laser deposition (PLD) [2]. In this work we study the optical properties and transport in gallium nitride (GaN) deposited by this technique. Optical spectroscopy (transmission and photoluminescence measurements) and photoconductivity data are presented.