Localization Effect in Photoelectron Transport Induced by Alloy Disorder in Nitride Semiconductor Compounds

Near-band-gap photoemission spectroscopy experiments were performed on $p\text{−}\mathrm{GaN}$ and $p\text{−}\mathrm{InGaN}/\mathrm{GaN}$ photocathodes activated to negative electron affinity. The photoemission quantum yield of the InGaN samples with more than 5% of indium drops by more than 1 order of magnitude when the temperature is decreased while it remains constant for lower indium content. This drop is attributed to a freezing of photoelectron transport in $p$-InGaN due to electron localization in the fluctuating potential induced by the alloy disorder. This interpretation is supported by the disappearance at low temperature of the peak in the photoemission spectrum that corresponds to the contribution of the photoelectrons relaxed at the bottom of the InGaN conduction band.

Figure 1

(a) Calculated band structure in real space of the $\mathrm{InGaN}/\mathrm{GaN}$ heterostructures showing the conduction band minimum (CBM) and valence band maximum (VBM). The different photoemission processes are indicated by colored arrows. With below band gap excitation, electrons can be photoemitted from the near-surface BBR. With above band gap excitation, electrons photoexcited in the InGaN layer or in the underlying GaN layer can be photoemitted. (b) and (c) Excitation spectra of the QY measured on the $p\text{−}\mathrm{GaN}$ and ${\mathrm{In}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{N}/\mathrm{GaN}$ samples, respectively. The positions of ${\mathrm{In}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{N}$ and GaN gaps at 300 K are indicated. The color of each spectrum indicates the temperature at which it was acquired according to the color scale shown in inset.

Figure 2

(a) Schematics of the photoemission process. The EDC is delimited at low energy by the vacuum level $E_{\mathrm{vac}}$ and at high energy by the final state of the optical transition from $E_F$. A characteristic low-energy peak labeled $\mathrm{\Gamma }$ is formed by photoelectrons which accumulate at the bottom of the conduction band in the bulk semiconductor and partially relax their energy in the BBR before emission. (b) EDCs measured at different temperatures between 140 and 300 K on $p\text{−}{\text{In}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{N}$, with $h\nu =3.65\mathrm{eV}$. The color of each spectrum indicates the temperature at which it was acquired according to the color scale shown in the inset.

Figure 3

Variation of the QY versus temperature, at a fixed, above-band-gap excitation energy: 2.9, 3.3, 3.45, and 3.5 eV, respectively, for ${\mathrm{In}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{N}$, ${\mathrm{In}}_{0.05}{\mathrm{Ga}}_{0.95}\mathrm{N}$, ${\mathrm{In}}_{0.02}{\mathrm{Ga}}_{0.98}\mathrm{N}$, and GaN.