Different intensities and wavelengths of nanosecond Nd:YAG laser were used to irradiate the samples (pulse repetition rate 12.5Hz, power P = 1.0MW). The laser beam to the irradiated surface of the samples was directed normally. The diameter of the spot of the laser beam was 3mm, and point-to-point method was used for irradiation of the samples. All experiments of nanocone formation were performed at atmospheric pressure, T = 20°C, and 60% humidity.

Current-voltage (I-V) characteristics were measured for the non-irradiated and irradiated samples. Measurements of I-V characteristics were performed by soldering 99% tin and 1% antimony alloy contacts directly on the irradiated surface of Ge and tin contacts on the opposite side. Measurements of I-V characteristics were done at room temperature and atmospheric pressure. Rectification ratio (RR) of I-V characteristics was used for the characterization of p-n junction.

Investigation of the process of nanocone formation by different laser radiation intensities, using multimode laser, has shown a non-uniform deposition of Cu on p-Si substrates. After irradiation by multimode laser, laser-induced periodic surface structures are formed on the surface of Si. Cu was deposited on the top of periodic structures forming lines (see Figure 2).

It is known that Cu can be electrochemically deposited on n-type semiconductor [12]. Therefore, it was supposed that inversion of conductivity type takes place, and 'hills' of the periodic structures are of n-type. Possibility to invert conductivity type by laser radiation was shown in several p- and n-type semiconductors: p-Si [13-15], p-CdTe [16], p-InSb [17,18], p-InAs [19], p-PbSe [20], p-Ge [21], and n-HgCdTe [22]. Different mechanisms have been proposed to explain the nature of inversion of conductivity type, for example, impurities' segregation, defects' generation [20], amorphization [23] and oxygen-related donor generation [14]. However, there are many contradictions in the mechanisms. For example, n-type impurities in Si irradiated by laser cannot be oxygen atoms, according to paper [15]. Several authors have tried to explain p-n junction formation in n-type HgCdTe by defects’ generation, based on a model of defect formation related to interstitial mercury diffusion [24]. On the other hand, the authors of those papers did not take into account the effect of temperature gradient on the diffusion of atoms in solid solution. Moreover, it is theoretically shown that the p-n junction can be formed by redistribution of impurities in co-doped Si in gradient temperature field [25]. Thus, the mechanism of inversion of conductivity type by laser radiation is not clear until now.

I-V characteristics of i-Ge samples before and after irradiation by Nd:YAG laser with a wavelength of 266nm and different laser intensities are shown in Figure 3. The I-V characteristic of the non-irradiated sample is linear. It means, I-V characteristics obey Ohm’s law, and therefore, there are no potential barrier between the electric contacts and the sample. After irradiation by the laser, I-V characteristics become diode like. Moreover, this process takes place in threshold manner, it means, RR is non-monotonic function on laser radiation intensity. These results are explained by damage of p-n junction at threshold intensity (I_th) due to formation of nanocones by laser radiation on a surface of semiconductor. A more detailed investigation of RR as a function of irradiation intensity of Nd:YAG laser for different wavelengths of the laser radiation and 350 laser pulses is shown in Figure 4. Threshold intensities are observed at the fundamental frequency I_th1 = 5.5MW/cm², the second harmonic I_th2 = 1.5MW/cm², and the fourth harmonic I_th4 = 1.15MW/cm², as shown in Figure 4 - the dashed line. We can see that RR and I_th are decreasing with increase of laser intensity and laser wavelength. We explain such behavior of I-V characteristics by formation of p-n junction at different depths which depends on penetration depth of laser beam. The decrease of the threshold intensity with increase of wavelength of Nd:YAG laser radiation and appearance of current-voltage characteristic rectification effect are explained in the following way: p-n junction is formed as a result of generation and redistribution of intrinsic point defects - vacancies and interstitial atoms in temperature gradient field - thermogradient effect [26], which is caused by strongly absorbed laser radiation. According to thermogradient effect, interstitial atoms drift towards the irradiated surface, but vacancies drift to the opposite direction - in the bulk of semiconductor. Since the interstitials in Ge crystal are of n-type and vacancies are known to be of p-type [27], a p-n junction is formed. Schematic illustration of n-p-i structure after irradiation of i-Ge sample is shown in Figure 5. We can see that shallow donor levels in band diagram according to the paper [28] has E_d = 0.04eV, and deep acceptor level has E_a = 0.2eV. The presence of deep acceptor level could be the main reason of low RR for p-n junction formed by laser radiation. For the improvement of I-V characteristics, it is necessary to use an acceptor with shallow level, for example Sb.

The mechanism of p-n junction formation by laser radiation in elementary semiconductors is the first stage of nanocone formation.

The second stage of nanocone formation in elementary semiconductors is laser heating up of the top layer enriched by interstitial atoms with further plastic deformation due to compressive stress caused by interstitials in the top layer and vacancies in the buried layer.