Origin of giant photocontraction in obliquely deposited amorphous ${\text{Ge}}_x{\text{Se}}_{1-x}$ thin films and the intermediate phase

Obliquely deposited amorphous ${\text{Ge}}_x{\text{Se}}_{100-x}$ thin films at several compositions in the $15\%<x<33.3\%$ range and at several obliqueness angles in the $0<\alpha <80°$ range at each $x$ were evaporated on Si and glass substrates. Here $\alpha$ designates the angle between film normal and direction of vapor transport. Raman scattering, IR reflectance, and optical absorption measurements were undertaken to characterize the vibrational density of states and optical band gaps. Edge views of films in scanning electron microscopy (SEM) confirm the columnar structure of obliquely $\left(\alpha =80°\right)$ deposited films. Films, mounted in a cold stage flushed with ${\text{N}}_2$ gas, were irradiated to UV radiation from a Hg-Xe arc lamp, and photocontraction (PC) of oblique films were examined. Compositional trend of PC exhibit a bell-shaped curve with a rather large effect $\left(0.25\mu \text{m}\right)$ centered in the $20\%<x<25\%$ range, the intermediate phase (IP) with the PC decreasing at $x>25\%$, the stressed rigid phase, and at $x<20\%$, the flexible phase. IR reflectance confirmed absence of photo-oxidation of films under these conditions. The IP represents a range of compositions across which stress-free networks form. Columns observed in SEM reveal a high aspect ratio, with typical lengths in the $1–2\text{−}\mu \text{m}$ range and a lateral width in the 50-nm range. We observe a blueshift (up to 0.38 eV) in the optical band gap of oblique films $\left(\alpha =80°\right)$ in relation to normally deposited $\left(\alpha =0°\right)$ ones, a result we identify with carrier confinement in nanofilaments $\left(<10\text{nm}\right)$ that form part of columns observed in SEM. In the IP, the large PC results due to the intrinsically stress-free character of filaments, which undergo facile photomelting resulting in film densification. Ge-rich films $\left(25\%<x<33.3\%\right)$ are intrinsically nanoscale phase separated and consist of nanofilaments $\left(\sim {\text{Ge}}_{25}{\text{Se}}_{75}\right)$ that demix from a Ge-rich $\left(\sim {\text{Ge}}_{40}{\text{Se}}_{60}\right)$ phase that fills the intercolumnar space. Loss of PC in such films is traced to the growth of the Ge-rich phase, which is intrinsically stressed and photoinactive. In contrast, Se-rich films are homogeneous, and loss of PC as $x$ decreases below 20% is traced to the accumulation of network stress in the Se-rich nanofilaments. The microscopic origin of the giant PC effect in amorphous semiconducting thin films can be traced, in general, to three conditions being met: (i) growth of a columnar structure leading to porous films, (ii) formation of columns that are rigid but intrinsically stress free, and (iii) an appropriate flux of pair-producing radiation leading to photomelting of columns. These findings lead us to predict that PC will, in general, be maximized in obliquely deposited films of semiconducting networks glasses residing in their IP when irradiated with supra-band-gap radiation.

Figure 1

(Color online) Schematic of the cold stage (top and side views) used to examine photocontraction effects in obliquely deposited films. Films are mounted onto a sample mount, located in a chamber that is thoroughly purged by high-purity dry ${\text{N}}_2$ gas (ports C and ${\text{C}}^{\prime }$). The cold stage is cooled (ports A and ${\text{A}}^{\prime }$) by a continuous flow of water. The sample mount could also be directly cooled (ports B and ${\text{B}}^{\prime }$). D: Thermocouple, E: sample mount heater, S: sample, w: optical window. Sample temperature was monitored during irradiation of films. See text for details.

Figure 2

Profilometric scans of thin films in their virgin state (a) $x=15\%$, (c) 22%, and (e) 33.3%. Film thickness reduction following irradiation shows a step as illustrated for films at (b) $x=15\%$, (d) 22%, and (f) 33%. Film thickness could be measured to about 5 nm accuracy with the profilometer.

Figure 3

(Color online) Thickness changes of ${\text{Ge}}_x{\text{Se}}_{1-x}$ thin films after photocontraction (●) showing a bell-shaped curve with a broad maximum near $x\sim 23\%$. The nonreversing enthalpy at $T_g$ of corresponding bulk glasses $\left(\Delta H_{\text{nr}}\right)$ (diamond symbol) reveals a global minimum in the $20\%<x<25\%$ range identified as the intermediate phase. See text for details.

Figure 4

(Color online) SEM images of (top) normally $\left(\alpha =0°\right)$ and (bottom) obliquely $\left(\alpha =80°\right)$ deposited amorphous ${\text{Ge}}_x{\text{Se}}_{1-x}$ thin films $\left(x=33.3\%\right)$. Columnar growth in the obliquely deposited film is observed as indicated by the arrows. The $1\text{−}\mu \text{m}$ bar sets the scale in the top panel, and the 250 nm bar in the bottom panel.

Figure 5

(Color online) SEM images of (top) normally $\left(\alpha =0°\right)$ and (bottom) obliquely $\left(\alpha =80°\right)$ deposited amorphous ${\text{Ge}}_x{\text{Se}}_{1-x}$ thin films $\left(x=23\%\right)$ on glass slide. Columnar growth of the obliquely deposited film is observed in the bottom panel, with a lateral size of column of about 50 nm. The $1\text{−}\mu \text{m}$ bar sets the scale.

Figure 6

(Color online) SEM images of (top) normally $\left(\alpha =0°\right)$ and (bottom) obliquely $\left(\alpha =80°\right)$ deposited amorphous ${\text{Ge}}_x{\text{Se}}_{1-x}$ thin films $\left(x=27\%\right)$ on glass slide. Columnar growth of the obliquely deposited film is observed in the bottom panel. The $1\text{−}\mu \text{m}$ bar sets the scale.

Figure 7

(Color online) Raman scattering of bulk ${\text{GeSe}}_2$ glass and amorphous films at indicated angle $\left(\alpha \right)$ of deposition. Note that the normally deposited $\left(\alpha =0\right)$ film shows a Raman spectrum similar to that of the glass used as the evaporation charge. Raman line shapes change systematically as $\alpha$ increases and permit network structure to be decoded. See text.

Figure 8

Raman scattering of bulk ${\text{Ge}}_{30}{\text{Se}}_{70}$ glass, normally $\alpha =0$) and obliquely deposited films at $\alpha =45°$, $60°$, and $80°$, showing a pattern similar to the one seen for ${\text{GeSe}}_2$ films in Fig. 4.

Figure 9

Raman scattering of bulk ${\text{Ge}}_{23}{\text{Se}}_{77}$ glass, normally deposited $\left(\alpha =0\right)$, and obliquely deposited films at $\alpha =60°$ and $80°$, showing little or no change as a function of $\alpha$.

Figure 10

(Color online) Results of Raman line-shape analysis yielding variations in (a) $A_1$ mode frequency; (b) $A_1^{\text{c}}$ mode frequency and (c) Raman intensity ratio of $A_1^{\text{c}}/A_1$ as function of obliqueness angle. Results for amorphous ${\text{Ge}}_x{\text{Se}}_{1-x}$ thin films at indicated compositions “$x$” are plotted.

Figure 11

Results of Raman line-shape analysis on bulk ${\text{Ge}}_x{\text{Se}}_{1-x}$ glasses, showing plots of (a) $A_1$ mode frequency; (b) $A_1^{\text{c}}$ mode frequency; (c) Raman intensity ratio of $A_1^{\text{c}}/A_1$; (d) average frequency of ${\text{Se}}_{\text{n}}$ chain mode; and (d) Intensity ration of ${\text{Se}}_{\text{n}}$ chain mode to $A_1$ mode as function of glass composition $x$. These parameters help in fixing the stoichiometry of the backbone in corresponding films.

Figure 12

(Color online) Micro-Raman spectra of obliquely deposited $\left(\alpha =80°\right)$ ${\text{GeSe}}_2$ films taken before (black) and after (red or gray) photocontraction at several depths “$d$” using a confocal microscope. See text for details.

Figure 13

(Color online) UV-VIS absorption of normally deposited ${\text{Ge}}_x{\text{Se}}_{1-x}$ films examined as a function of photon energy. Plots of (a) optical absorbance, (b) optical absorption coefficient, $\mu$, and (c) of ${\left(\mu h\omega \right)}^{1/2}$ as a function of photon energy appear in different panels. Optical band gaps of films increase as a function of “$x$” as illustrated in Figure 11.

Figure 14

(Color online) Optical band gap of normally deposited ${\text{Ge}}_x{\text{Se}}_{1-x}$ films. (a) The filled circles give the gap $E_{53}$, measured at $\mu =5\times {10}^3{\text{cm}}^{-1}$, while the open circles are data of Street et al. (Ref 37) taken at $\mu =1\times {10}^3{\text{cm}}^{-1}$. (b) Filled circles give the Tauc edge, $E_T$, of the present films as a function of $x$. Filled and open squares represents, respectively, $E_T$ of amorphous $x=33.3\%$ thin films in their as deposited and after optical annealing taken from the work of Spence and Elliott (Ref. 9).

Figure 15

(Color online) Plots of (a) UV-VIS absorbance and (b) absorption coefficient $\mu$ of ${\text{Ge}}_x{\text{Se}}_{1-x}$ thin films at $x=22\%$ as a function of photon energy for films at indicated obliqueness angle $\alpha$. Plots of (c) ${\left(\mu h\omega \right)}^{1/2}$ against $E$, and (d) variations in the optical band gaps $E_{53}$ and $E_T$ of $x=22\%$ films as a function of obliqueness angle.

Figure 16

(Color online) Plots of (a) UV-VIS absorbance and (b) absorption coefficient $\mu$ of ${\text{Ge}}_x{\text{Se}}_{1-x}$ thin films at $x=23\%$ as a function of photon energy for films at indicated obliqueness angle $\alpha$. Plots of (c) ${\left(\mu h\omega \right)}^{1/2}$ against $E$, and (d) variations in the optical band gaps $E_{53}$ and $E_T$ of $x=23\%$ films as a function of obliqueness angle.

Figure 17

(Color online) Plots of (a) UV-VIS absorbance and (b) absorption coefficient $\mu$ of ${\text{Ge}}_x{\text{Se}}_{1-x}$ thin films at $x=30\%$, as a function of photon energy for films at indicated obliqueness angle $\alpha$. Plots of (c) ${\left(\mu h\omega \right)}^{1/2}$ against $E$, and (d) variations in the optical band gaps $E_{53}$ and $E_T$ of $x=30\%$ films as a function of obliqueness angle.

Figure 18

(Color online) Optical band gap $E_{53}$ of normally and obliquely deposited ${\text{Ge}}_x{\text{Se}}_{1-x}$ thin films as a function of “$x$.” Note the much higher value of the gap in obliquely deposited films.

Figure 19

(Color online) IR absorbance of (a) ${\text{GeO}}_2$ glass and amorphous ${\text{Ge}}_{22}{\text{Se}}_{78}$ films before and after irradiation, and (b) ${\text{GeO}}_2$ glass and amorphous ${\text{Ge}}_{23}{\text{Se}}_{77}$ films before and after irradiation. In (a) the cold stage was thoroughly flushed with ${\text{N}}_2$ gas while in (b) this was not the case. Note the presence of oxidation in the 23% film in (b) but the absence of it in the 22% film in (a).

Figure 20

(Color online) Simulation of the observed Raman line shape of an obliquely $\left(\alpha =80\right)$ deposited ${\text{GeSe}}_2$ film in terms of two contributions, one of ${\text{Ge}}_{25}{\text{Se}}_{75}$ bulk glass (A nanophase) and the other of ${\text{Ge}}_{36}{\text{Se}}_{64}$ bulk glass (B nanophase), revealing nanoscale phase separation of such films. See Eq. (2) in text.