A Notes

Jones RE, et al.

Evidence for p-Type Doping of InN.
PRL 96, 125505 (2006).

Ref. 0 > doi: 10.1103/PhysRevLett.96.125505

InN has attracted great interest due to its potential applications in optics and electronics. After several years of debate, it has been demonstrated recently that the band gap of InN is only about 0.7 eV,^CR1,CR2 not 1.9 eV, which is in good agreement with the theoretical prediction.^CR3 Note that GaN and AlN are two kinds of semiconductor with large band gaps, III-nitrides potentially cover the spectral range from infrared to ultraviolet. The narrow band gap has generated great interest in InN for applications such as high-efficiency solar cells, light-emitting diodes, laser diodes, and high-frequency transistors. The ability to fabricate both p-type and n-type InN is essential to the realization of these devices; however, only n-type InN has been reported till the publication of the work referenced (Ref. 0). 

Before we get into this work, we can take a look at two important theoretic studies on InN from University of Warwick to get familiar with two basic characteristics of InN: electron accumulation on the surface^CR4 and n-type.^CR5  

Firstly, I. Mahboob and his co-workers^CR4 for the first time discussed the electron accumulation in the surface.

Fig.1 (Ref. 4^CR : PRL 92, 036804 (2004).)

Mahboob I, Veal TD, McConville CF, Lu H & Schaff WJ,
Intrinsic Electron Accumulation at Clean InN Surfaces.
Phys. Rev. Lett. 92, 036804 (2004).

Reproduced by Scidea Art 2007. Source: ScideaNews.com
HRmage Fig.01

Fig.2 (Ref. 5^CR : APL 88, 252109 (2006). )

Piper LFJ, Veal TD, McConville CF, Lu H & Schaff WJ,
Origin of the n-type conductivity of InN: The role of positively charged dislocations.
Appl. Phys. Lett. 88, 252109 (2006).

Reproduced by Scidea Art 2007. Source: ScideaNews.com
HRmage Fig.02

Fig.3 (The topic: Jones RE, et al. PRL 96, 125505 (2006). doi: 10.1103/PhysRevLett.96.125505)

Jones RE,Yu KM, Li SX, Walukiewicz W, Ager JW, Haller EE, Lu H & Schaff WJ,
Evidence for p-Type Doping of InN.
Phys. Rev. Lett. 96, 125505 (2006).

Reproduced by Scidea Art 2007. Source: ScideaNews.com
HRmage Fig.03

Let's get back to the work we referenced. Fig. 3 demonstrates the most important result of their discovery. In nominally undoped (n-type) InN, the electron concentration has a maximum near the surface (electron accumulation layer) and rapidly decreases deeper into the film, appearing to saturate at a value close to the bulk electron concentration measured by Hall effect. Mg doped InN has an electron accumulation layer similar to undoped films at the surface, but it was also found that the Mg-doped InN indicates p-type during form 6–8 nm from the surface. 

With this significant discovery, they also did some other job in this work. (Irradiation with 2 MeV He-ions is used to convert the bulk of InN:Mg from p- to n-type, at which point photoluminescence is recovered). The conversion is explained by a model assuming two parallel conducting layers (the surface and the bulk) in the films.

At last, we have to point out that the measurement of net charge density in dependence of depth was via C-V investigation, with NaOH etching. However, one of the PRL's authors H. Lu once talked to me that a more credible method to prove p-type is "hot probe method". (That's only a talk, still no work yet).

In my opinion, this work is significant, because they demonstrate p-type of Mg:InN below the n-type surface, that makes InN p-n junctions possible. However, to achieve p-n juctions and other InN-based devices, I believe the most severe problem to be solved is the large surface state density of InN which makes the contact of InN to other materials difficult. □

Comments in this issue >>>>>>